1. Technical Field
The present invention relates to a rotary position measuring instrument.
2. Background Information
FIG. 1 shows a schematic fragmentary view of a known optical position measuring instrument that is based on a central-projection scanning principle. A light source 1 divergently illuminates a reflective measuring standard 2. The measuring standard can be embodied as either a linear or a rotary graduation, depending on whether the position measuring instrument is to be used for detecting rotary or linear relative motions. From the measuring standard 2, incident beams are reflected back in the direction of the light source 1. In a detection plane, the incident beams strike an optoelectronic detector assembly 3. The measuring standard 2 is movably disposed relative to the light source 1 and the detector assembly 3. In other words, the measuring standard 2 is either displaceable along a linear axis or rotatable about an axis of rotation. In the case of the relative motion of the measuring standard 2 relative to the light source 1 and the detector assembly 3, the result in the detection plane is a modulated fringe pattern, which can be converted, via the detector assembly 3, into motion-dependent position signals.
With the aid of a position measuring instrument of this kind, the relative or absolute position of two objects movable (linearly or rotationally) relative to one another can be determined. One of the two objects is connected to the measuring standard 2, and the other object is connected to the light source 1 and the detector assembly 3.
As can be seen from FIG. 1, the light source 1 is spaced apart from the measuring standard 2 at a distance u that will hereinafter be called the first distance. The detector assembly 3 is spaced apart from the measuring standard 2 by a distance v, which will hereinafter be called the second distance. In FIG. 1, the second distance v is different in magnitude from the first distance u.
In terms of a model, the scanning beam path described can also be viewed such that instead of the real light source 1 at the distance u from the measuring standard 2, a virtual light source 1′ at a distance u′, wherein |u′|=|u|, illuminates the measuring standard 2 from the other side. The beams emitted by the virtual light source 1′ effect a central projection of the scanned graduation of the measuring standard 2 into the detection plane.
Rotary position measuring instruments based on an optical central-projection scanning principle are known, for instance from Japanese patent disclosure JP 9-133552 A. That position measuring instrument includes a light source, a graduated disk with a rotationally symmetrical, reflective measuring standard, and an optoelectronic detector assembly. The graduated disk is rotatable relative to the light source and the detector assembly about an axis of rotation, so that in the event of the relative rotation, rotary-angle-dependent position signals are detectable via the detector assembly.
The central-projection scanning principle employed for generating the position signals is also known as a three-grating scanning principle; information on which can be found for instance in the publication by R. Pettigrew entitled “Analysis of Grating Imaging and its Application to Displacement Meteorology”, in SPIE, Vol. 36, 1st European Congress on Optics Applied to Meteorology (1977), pp. 325-333.
In the ideal case, in such a central-projection scanning principle the magnitudes of the first distance u and the second distance v are selected to be identical. To that end, the corresponding components of the position measuring instrument should be suitably placed in the scanning beam path. With identically selected distances u, v, any changes in the distance between the objects, or between the measuring standard 2 and the light source 1/detector assembly 3, that is, in the scanning distance, do not have an effect on the size of the fringe pattern that results in the detection plane. In practice, however, it is not always ensured that the first and second distances u, v are identical. That is, often a situation like that shown in FIG. 1 results in which the magnitude of the first distance u is different from the magnitude of the second distance v. In the example of FIG. 1, wherein u<v, fluctuations in the scanning distance cause changes in the size of the fringe pattern and, thus, cause errors in the position determination.
Further problems in such position measuring instruments result from their sensitivity to soiling on the measuring standard.